Great interest has recently developed in supplementary methods of charging the battery of an automobile which can be efficiently carried out with a battery charger. This battery charger incorporates a solar cell accommodated in the body of a case in a manner such that one surface of the solar cell is located in an opening formed in one surface of the case body. In addition, one end of a cable is electrically connected to the solar cell. The other end of the cable is provided with a plug which can be connected to a cigarette lighter socket located in the dashboard of an automobile.
In this battery charger the electric potential of the secondary battery is higher than that of one solar cell. Because of this a multiplicity of solar cells need to be connected in series to produce sufficiently large voltage. However, the manufacturing costs are extremely high when monocrystal type or polycrystal type solar cells are used.
Production costs can be reduced when an amorphous silicon solar cell is employed. However, in this case the weatherability and heat-resistance of the battery charger are not sufficient. The reason for this is that the place where the battery charger is positioned, such as the top surface of the dashboard of an automobile, becomes very hot at certain times of the year, for example during the summer. Metallic components which form the reverse electrode of an amorphous solar cell are in the presence of heat, prone to diffuse into a semiconductor layer. This leads to the deterioration of the semiconductor layer.
Another type of battery charger is known in which the secondary battery is supplementarily charged with solar cell. In this case, the electric potential of the secondary battery is displayed, as shown in FIG. 11, and the solar cell a is connected to an anode of the secondary battery d through an LED b and an inversion preventing diode c.
The only object of such a battery charger is to charge the secondary battery, and the LED b can therefore display the state in which the secondary battery d is being charged. However, the electric potential of the secondary battery d can neither be measured nor displayed. This results in the LED b being lit up and the secondary battery d being charged by means of the solar cell a, and the capacity of the secondary battery being almost empty. The supplemental charging performed by the solar cell cannot compensate for this and why the secondary battery cannot be used is not clearly understood.
Further, such a battery charger completely lacks a fastening member for securing it to an automobile. Instead, a very well known method of securing the battery charger to an automobile is to provide a hook, this hook is itself secured to a sucker which is adapted to be secured to the windshield of an automobile. Alternatively, a double-sided tape is sometimes employed for the purpose of securing the battery charger to an automobile.
The solar cell side of the charger needs to face upward when placed on the top surface of the dashboard of an automobile so that it may efficiently be exposed to the sun. However, when the battery charger completely lacks a fastening member, the battery charger often moves or falls as a consequence of sudden movements of the vehicle, e.g., when starting or stopping or some other form of shock occurring during driving. Accordingly, the solar cell cannot always be maintained facing the sun.
In addition, other problems arise when the battery charger is secured to a hook fixed to a sucker which is, in turn, secured to a windshield. In this case, the battery charger moves or swings with respect to the hook when any impact or vibration is experienced during the movement of the automobile. Thus, the solar battery cannot always face the sun, plus there is a risk of cable breakage.
Further, when double-sided tape is used for fastening the charger, attachment and detachment thereof cannot be carried out freely, so shifting of the solar battery in an automobile when this is necessary during parking is difficult. The tape may also cause damage to the surface of the automobile to which it is attached.